Such multistep electronic cooler is, for example, formed in six steps as shown in FIG. 1A and contained in a vacuum bell jar 5, and on each step, a thermoelectric element A comprising a pair of N-type semiconductor 1 and a P-type semiconductor 2 is connected via an electrode 4 bonded to a base plate 3. The cooler is adapted such that, by passing electric current through the thermoelectric elements A on each step, the side of the base plate at the upper side of each step is deprived of heat, and thereby, the base plate (cooling plate) on the uppermost step, 3a, is cooled to a maximum achievable cooling temperature.
At present, as an example of such multistep electronic coolers, there is a cascade six-step electronic coolers manufactured by Marlow Industries, Inc. performance of it is as shown in FIG. 1B.
That is, the relationship between the current I and the voltage V is proportional as shown by a in FIG. 1B when the degree of vacuum in the vacuum bell jar 5 containing the electronic cooler is 10.sup.-6 Torr and the outside air temperature T.sub.h =27.degree. C., the relationship of the cooling temperature T.sub.c against the current I changes along a parabolic curve shown by b in the figure when incoming heat quantity Q from outside the vacuum bell jar 5 is zero, i.e., Q=0, and the cooling temperature T.sub.c when the current=3.5 A reaches -100.degree. C. under the ideal condition of Q=0 as indicated by c in the figure, and then the cooling temperature T.sub.c rises proportionally as Q increases.
In such a multistep electronic cooler as described above, the maximum achievable cooling temperature T.sub.c under the ideal condition of the incoming heat quantity Q=0 becomes -100.degree. C. when the current I=3.5 A. In the past, however, since the cooler was just arranged to be contained in a vacuum bell jar 5, it easily suffered from the effect of the outside air temperature, and it never reached anywhere near the cooling temperature of -100.degree. C. because there was unavoidably present a certain incoming heat quantity Q.
More specifically, the incoming heat quantity Q to the multistep electronic cooler within the vacuum bell jar 5 is caused by both convective heat transfer in the vacuum bell jar 5 and radiant heat transfer from the outside. Of these heat transfers, the convex heat transfer is made noneffective by evacuating the bell jar 5. But, since the arrangement is only such that the multistep electronic cooler is contained in a vacuum bell jar 5, it was unable to prevent the external radiant heat from coming in and, hence, it was unable to reduce to zero the incoming heat quantity Q due to the radiant heat transfer.